The present invention concerns a process for the purification of an organic product by fractionated crystallization at a variable circulation rate. The invention also pertains to an apparatus for the implementation of this process.
Known in the prior art are three types of processes for the purification of organic products by crystallization:
Continuous crystallization in which the crystals are formed in a crystallizer and maintained in suspension. The slurry of crystals is then sent to a separation system that is generally constituted of a centrifuge or washing column.
Static crystallization which is a discontinuous process employing an apparatus equipped with a heat transfer device. This apparatus is first filled with the crude product which is completely liquid at that point. The apparatus is then cooled in order to generate crystals (crystallization step). These crystals stick to the walls. When a sufficient quantity of crystals has been formed, the liquid charged with impurities is drained off gravitationally (drainage step). In order to increase the purity of the product remaining in the apparatus, a partial melting of the crystals can be implemented (purification of the residual liquid). This is the liquid phase separation step. The crystals are then melted so as to obtain the purified product in liquid form (melting step). All of these steps together constitute a stage. One generally speaks of a purification stage when the previously mentioned steps are intended to purify the product and of a concentration stage when the steps are intended to concentrate the impurities (improvement of the yield). Attached FIG. 1 shows the recyclings that are implemented when multiple stages are required. In the case of the static process, the crystals can grow in the entire volume of the crystallizer. At the end of the crystallization step, they constitute a network of relatively constant porosity. During the drainage step, the liquid flows through this network. It is therefore recommended that the height of the apparatus be limited so that the drainage step can be implemented within an acceptable period of time.
Dynamic crystallization which is also a discontinuous process; the phases of the process are identical to those of static crystallization. The principal difference is that the product circulates in the apparatus during the crystallization phase. In the case of the dynamic process, the product which crystallizes circulates inside tubes. Because of the movement of the product, the crystallization takes place in layers. At the end of the crystallization step, each tube therefore contains a layer of crystals sticking to the walls, and a central well containing the waste liquid. The drainage step consists of draining these central wells. This is a brief step (several minutes) and the apparatus can be of any height without having a negative effect on the efficiency of the step.
The continuous crystallization process has the major disadvantages of generating a slurry of crystals, which is often difficult to transport or pump, and to require many mechanical devices (agitators, surface scrapers, centrifuges, etc.) which can create maintenance problems.
The static and dynamic crystallization processes are therefore preferred since all of the product transfers are implemented in liquid phase and the only mechanical devices employed are pumps.
In addition, dynamic crystallization has the advantage of short cycle times compared to static crystallization, which can lead to economic advantages in certain cases. In contrast, the purification factor per stage is often less favorable. In addition, certain products can not be purified using this process when their impurity content is too high.
It has also been proposed in the prior art that the static and dynamic processes be combined. In effect, the purity required of certain products necessitates purification in multiple stages. The operating yield also necessitates multiple concentration stages.
When multiple stages are necessary and when the volume to be treated is large, the use of the dynamic process can be advantageous from the investment point of view (smaller installed volume than in the case of the static process).
Nevertheless, this use of the dynamic process is sometimes only possible for purification stages in which the content of impurities is low. When the impurity content becomes too high, the form and the size of the crystals do not enable good adherence to the walls and the crystals are carried along by the circulating product.
It is for this reason that the static and dynamic processes are sometimes combined:
use of the dynamic process in the purification stages (and sometimes in the initial concentration stages) in order to reduce the installed volume,
use of the static process in the final concentration stages, since the liquid is not moving which allows the crystals to stick correctly to the wall and enables solid/liquid separation.
Nevertheless, the combination of the static and dynamic processes has two major drawbacks:
In the dynamic processes, the circulation rate of the product is fixed either by gravity in the case of a falling film process such as proposed in European patent no. 616 998, or by the pump circulation rate in the case of a filled tube process. The drawback of these processes is that the circulation rate is never the rate that best corresponds to the impurity content of the product.
The necessity of switching from the dynamic process to the static process occurs when, for a stage, the circulation rate no longer allows correct adherence of the crystals to the wall. Switching over to the static process gives rise to multiple disadvantages: on the one hand, the volume of certain stages is then larger than if that stage were operating at a reduced circulation rate and, on the other hand, two different processes are implemented which increases the complexity of the installation.
The goal of the present invention is to resolve these disadvantages by providing a purification process that makes it possible to take into account the impurity content of the product and which is simple to implement. This goal is attained by means of a process for the purification of an organic product by fractionated crystallization, characterized in that the organic product is purified by circulation in filled tubes by varying the circulation rate at each purification and concentration stage.
The present invention also pertains to an apparatus for the implementation of the above-described process. This apparatus is of the type comprising at least one tube type crystallizer, constituting a tube zone through which the product current passes completely, tanks for feeding the apparatus with crude product, circulation and drainage of the intermediate and purified products, lines for the transport of the products from the tanks to the crystallizer(s) and vice versa, valves that make it possible to put the tube zones in series or to allow drainage of each tube zone, for adjusting the product circulation rate for each stage and circulation pumps equipped with a velocity variator.